Currently, polyarylene sulfide is representative engineering plastic, and is in great demand for high temperature and corrosive environment and electronic products due to high heat resistance, chemical resistance, flame resistance, and electric insulation. The major uses include computer accessories, automobile accessories, coating of the part contacting corrosive chemical materials, and industrial chemical resistant fiber, and the like.
Presently, polyphenylene sulfide (PPS) is the only commercially available polyarylene sulfide. According to the current commercial production process of PPS, p-dichlorobenzene (pDCB) and sodium sulfide are used as raw materials and reacted in a polar organic solvent such as N-methylpyrrolidone, and the like. This method is known as a Macallum process, and the basic process is disclosed in U.S. Pat. Nos. 2,513,188 and 2,583,941, wherein there are some usable polar solvents but N-methylpyrrolidone is currently most commonly used. This process uses dichloro aromatic compounds as raw material, and sodium chloride (NaCl) is generated as a by-product.
PPS obtained in the Macallum process has a molecular weight of about 10,000˜40,000, and the melt viscosity is 3000 Poise or less, which is not so high. In general, to obtain higher melt viscosity, PPS is subjected to a curing process wherein it is contacted with oxygen while heating below melting point (Tm), and thereby, melt viscosity may be increased to required level by oxidation, crosslinking, polymer chain extension, and the like.
However, PPS produced by the existing process has fundamental disadvantages as follows.
First, sodium sulfide, and the like is used for supplying sulfur required in the polymer reaction, and thus, a large quantity of metal salts such as sodium chloride, and the like exist in the polymer as by-products. For this reason, even after the polymer obtained in the process is washed, metal salts remain in several thousands ppm level, thus increasing electric conductivity of the polymer, inducing corrosion of processing devices, and raising problems in the spinning process when making fibers. And, if sodium sulfide is used as raw material, production amount of by-product sodium chloride reaches 52 wt % of the weight of the raw material, which becomes waste because the recovery is economically infeasible.
Second, properties of the polymer undesirably change in, the curing process. The color turns deep by crosslinking and oxidation by oxygen, and brittleness becomes higher.
Finally, similarly to the polymers obtained by solution polymerization, the final PPS becomes very fine powders, which relatively lowers apparent density thus rendering it unfavorable for transportation and causing a lot of inconveniences in the manufacture process.
In addition to the Macallum process, new processes are suggested in U.S. Pat. Nos. 4,746,758, 4,786,713 and relating patents. In these patents, diiodide compounds and solid sulfur are used instead of bichloride compounds and metal sulfide in the existing process, and it is disclosed that polyarylene sulfide may be obtained by direct heating without using a polar solvent. This method consists of two steps of iodination and polymerization, wherein in the iodination process, aryl compounds and iodine are reacted to obtain a diiodide compound, which is reacted with solid sulfur to prepare polyarylene sulfide having high molecular weight in the polymerization process. During the reaction, iodines are generated in the form of steam, which may be recovered to react with aryl compounds again, and thus, iodine is substantially a catalyst.
However, the following two problems are primarily indicated to be solved in this process. First, since iodine is corrosive if it remains in a molecular state, it may cause problems in processing devices if is included in the final polyarylene sulfide product. Second, because solid sulfur is used in the polymerization process, a disulfide link is included in the final polyarylene sulfide, which deteriorates thermal properties including melting point.
Accordingly, there is a demand for development of a process that does not generate metal salts unnecessary in the process, may remarkably reduce the content of iodine causing corrosiveness in devices, and may effectively prepare polyarylene sulfide having mechanical strength.